1. Field of the Invention
The present invention relates to a primary containment vessel for enveloping the core of a nuclear reactor constituted by a light water reactor of the boiling water type, and, more particularly, to a primary containment vessel which permits improvement of its inherent safety through a static cooling system replacing a pressure suppression pool water cooling system and of the economic efficiency by streamlining facilities and equipment.
In addition, the present invention relates to a natural heat-radiating type primary containment vessel which is suitable for cooling the primary containment vessel and/or reducing the emission of radioactive substances at the time of the loss of a coolant.
Furthermore, the present invention relates to a natural heat-radiating type primary containment vessel which is suitable for removing to outside the system by a natural force over an extended period of time thermal energy which is produced by core decay heat released to the primary containment vessel at the time of an emergency when the loss of a coolant has occurred.
The present invention also relates to a nuclear power plant provided with a condensate storage pool in a reactor building.
The present invention also relates to a primary containment vessel reinforcing ring which is suitable for cooling the inside of a primary containment vessel at the time of the occurrence of damage to the piping in the primary containment vessel.
Moreover, the present invention relates to a natural circulation-type nuclear reactor, and, more particularly, to a natural circulation-type nuclear reactor provided with an emergency reactor cooling system which is suitable for use in a boiling water reactor and capable of assuring the cooling of a core by such as maintaining the core immersing water over an extended period of time at the time of the occurrence of a loss-of-coolant accident and/or at the time of an emergency when control rods cannot be inserted.
2. Description of the Related Art
As an example of the prior art, there is a primary containment vessel having a pressure suppression of boiling water reactor facilities, as shown in FIG. 20.
A primary containment vessel 201 envelops a reactor pressure vessel 202, and the upper space therein surrounding the reactor pressure vessel 202 is called a dry well 203, while a container disposed at a lower portion thereof and filled with pool water 204 is called a pressure suppression 205.
The dry well 203 and the pressure suppression 205 are constructed such as to communicate with each other by means of a vent pipe 206. An open end of the vent pipe 206 is immersed in the water of a pressure suppression pool 204 stored in the pressure suppression 205.
In the dry well 203 are disposed the piping containing a high-temperature and high-pressure coolant, machines and instruments of a primary system of the reactor, in addition to the reactor pressure vessel 202. Furthermore, containment spray headers 207 for spray the cooling water are provided in the container 201.
In addition, a residual heat removal pump 208, a residual heat removal system heat exchanger 209 for removing residual heat, and piping from the pressure suppression pool to the spray header 207 via these machines are provided to supply the cooling water to the spray head 207. Furthermore, piping for returning the cooling water from the heat exchanger 209 for removing residual heat to the pressure suppression pool 204 is also provided. Incidentally, reference numeral 210 denotes a building constituting a biological shield.
If an emergency is assumed to have occurred in which the piping of the primary system of the reactor is fractured, the high-temperature, high-pressure coolant of the primary system of the reactor is released into the dry well 203, and a mixture of released steam and water is led to the pressure suppression pools 204 via the vent pipes 206. The released steam is cooled and condensed in the pressure suppression pools 204, thereby suppressing an internal pressure rise of the dry well 203.
When the efflux of the coolant from a fracture is completed, the high-temperature and high-pressure steam inside the primary containment vessel 201 is condensed by operating the spray headers 207, which causes the internal pressure of the primary containment vessel 201 to decrease rapidly.
When the water temperature of the pressure suppression pools 204 rises by the blow-down of steam, the pressure suppression pool water is cooled by the heat exchangers 209 for removing residual heat.
As described above, should the piping of the primary system of the reactor be fractured, when the accident takes place over a short period, the conventional primary containment vessel 201 attains the suppression of pressure by condensation of steam in the water of the pressure suppression pools 204. Meanwhile, when the accident takes place over a long period, the primary containment vessel 201 attains the suppression of pressure by condensation of steam by sprinkling from the spray headers 207 and inhibits a temperature rise of the water of the pressure suppression pool. Since the pressure suppression function in the pressure suppression pool 204 in the former case is constituted by the guiding function of the vent pipes 206 alone, this pressure suppression function is sufficient in ensuring inherent safety as well. On the other hand, to cool the primary containment vessel 201 over a long period of time and cool the pressure suppression pools 204, such dynamic machines as the residual heat removal system pumps 208, the heat exchangers 209, electrically-operated valves, etc., become necessary.
In the above-described conventional example, it has been necessary to retain in the pressure suppression pools a large quantity of water for cooling and condensing steam released at the time of a loss-of-coolant accident, and the heat exchangers for removing residual heat have been necessary for cooling the pressure suppression pools over a long period of time.
In addition, in the primary containment vessel of a boiling water reactor of the above-described conventional art, a residual heat removal system is provided to cope with the removal of core decay heat over a long period of time after the ECCS is operated subsequent to the accident of loss of the coolant and after the core is submerged with water. As a result, there have been drawbacks in that the costs become high, that the pool water containing fission products is led outside the primary containment vessel, and that it is troublesome to carry out, somewhat periodically, the operation test of dynamic machines such as pumps and heat exchangers to check the operation of the machines installed.
In contrast to the pressure suppression pool water-cooling system employing the configuration of dynamic machines and facilities such as the one described above, if as static a heat removing system as possible can be devised as a system having a similar cooling function in place of the facilities which dynamically function, such as rotary equipment, including pumps, large heat exchangers, and large piping loops, it is considered that substantial improvement will be made in the safety and reliability of the system per se through a reduction in the functional requirements for dynamic structural parts, and that the economic efficiency of the plant will be enhanced in conjunction with the streamlining of the facilities per se. As a prior art concerning a cooling system for a primary containment vessel employing such a static system, it is possible to cite a primary containment vessel cooling system based on a heat pipe system disclosed in, for instance, Japanese Unexamined Patent Publication No. (Japanese Patent Application Laid-Open (Kokai) No.) 125483/1980.
The arrangement of this system is such that a multiplicity of cylindrical heat pipes with a low-boiling-point liquid sealed therein are installed on the outer surface of a dry well steel plate of the primary containment vessel. This is a heat removing system in which heat retained in a gas inside the container dry well is statically allowed to escape to outside the primary containment vessel via these heat pipes. It is technically feasible to apply the heat pipes of this idea to the above-described water cooling system of the container pressure suppression pools, and a static safety cooling system can be arranged. However, since the heat pipes with the low-boiling-point liquid incorporated therein are installed as a large-scale structure, the heat pipes substantially affect as obstructions the layout of facilities installed on the outer biological shield wall of the primary containment vessel and an external space thereof. In addition, the structure of this system is bound to become large in size in view of the requirements of antiseismic design are an important facility relating to a safety system. Thus, this system has numerous problems in terms of its facilities, its structure does not have an economic advantage, with the result the system is not very realistic.
As another example of the prior art relating to a similar primary containment vessel cooling system, it is possible to cite a system disclosed in Japanese Unexamined Utility Model Publication No. 11689/1984 in which the primary containment vessel is filled with a liquid by filling the space between a primary containment vessel concrete wall and a liner with the liquid. In this system, however, the gap between the concrete wall and the liner is 2 mm or thereabout, and since the gap is too small to statically cool the pressure suppression pool water with the liquid of this pertinent portion, a circulating flow of the cooling external liquid does not occur. Consequently, this system is so unrealistic that a high static cooling efficiency cannot be obtained, and that it is impossible to expect its effect.
A conventional boiling water reactor plant disclosed in Japanese Unexamined Patent No. 137596/1979 has a condensate storage tank installed outdoors of a reactor building such as to be adjacent to the reactor building and a turbine building. This condensate storage tank is used as a water source for a fuel pool replenishing water system, and a control rod driving hydraulic system, as well as for adjustment of the holding water quantity. In addition, the condensate storage tank is also used as a water source for a cooling system at the time of isolation of the reactor as well as a high-pressure core spraying system, both of which are safety systems.
In a conventional boiling water reactor plant, the condensate storage tank is installed on an antiseismic foundation mat (concrete mat). For this reason, a large amount of concrete is required in structuring a special antiseismic mat described above, so that a long period of time has been necessary in constructing the entire foundation mat of a boiling water reactor plant.
As a structure for injecting cooling water into a primary containment vessel by making use of gravity at the time of a loss-of-coolant accident, a nuclear reactor disclosed in Japanese Unexamined Patent Publication No. 69289/1982 is proposed. As shown in FIG. 21, this reactor is arranged as follows: A core 211 is disposed in such a manner that a cooling water level 214 formed in a cooling water tank 213 is located above a cooling water level 212 formed in a reactor pressure vessel 202 incorporating the core 211. At the time when a loss-of-coolant accident has occurred, separation valves 215, 216 in a piping 214 communicating with a vapor phase portion of the reactor pressure vessel 202 and a cooling water tank 213 are opened to set the pressure of the two spaces at the same level. A pressure relief valve 218 is provided between a main steam pipe 217 and a pressure suppression 205. At the time of an accident when the coolant has been lost, steam inside the reactor pressure vessel 202 is released to a pressure suppression pool 204 via the pressure relief valve 218 so as to decrease the pressure. When the internal pressure of the reactor pressure vessel 202 has been decreased and dropped below that of the cooling water tank 213, separation valves 220, 221 of a piping 219 communicating with the bottom of the cooling water tank 213 and the reactor pressure vessel 202 are opened so as to supply the cooling water contained in the cooling water tank 213 into the reactor pressure vessel 202 by virtue of the operation of gravity.
In addition, the pressure suppression 205 is located above the cooling water level 212 formed inside the reactor pressure vessel 202 so as to be able to constitute an emergency core cooling system of the gravity dropping type. At the time of a loss-of-coolant accident, with the opening of a valve 222, the pool water 204 in the pressure suppression 205 is led into the reactor pressure vessel 202 by means of gravity. A similar structure of a nuclear reactor is disclosed at page 13 of Nuclear Engineering Vol. 31, No. 383 (June, 1986) as well.
This example of the prior art is provided with the pressure suppression 205 above the liquid level 212 inside the reactor pressure vessel 202. At the time of a loss-of-coolent accident, when the supply of the pool water 204 to the reactor pressure vessel 202 by the action of gravity is completed, there is a possibility that the pool water 204 inside the pressure suppression disappears. For this reason, the capabilities of condensing steam introduced by a vent pipe 223 disadvantageously decline.